1. Field of the Invention
The present invention relates to a thermoplastic material excellent in heat resistance and water resistance which comprises thermoplastic saturated norbornene polymer and more particularly, to a thermoplastic material which can provide articles excellent in strength and free from defects caused by blowing especially when it is extrusion formed.
The present invention further relates to a film-like or sheet-like article made from the above thermoplastic material and excellent in strength and free from fine defects.
Moreover, the present invention relates to an optical material made from the above thermoplastic material and excellent in transparency and an electroconductive composite material and an optical recording medium made from the above material.
2. Related Art
In general, polyolefinic polymers have such structure as consisting of only carbon and hydrogen and having no unsaturated bond and therefore, have the features that they are low in affinity with water and do not absorb or permeate water and besides, are high in resistance to acid, alkali and other chemicals or various solvents.
Representatives thereof are polyethylene and polypropylene and these are employed for various uses in the form of sheet, film, pipe, rod, and the like formed by melt extrusion. However, these conventional materials suffer from the problems that they are insufficient in heat resistance and become distorted at relatively low temperatures of 70.degree.-80.degree. C. or lower and cannot be used at a temperature higher than such temperature. Furthermore, these materials are low in transparency and cannot be used for optical purposes.
On the other hand, polycarbonate (PC) and polymethyl methacrylate (PMMA) have been mainly used as optical transparent plastic materials for optical disc substrate, plastic lens and the like. However, PC is high in birefringence and PMMA is high in water absorption and insufficient in heat resistance. Thus, these materials are not enough to meet the increasingly heightened demands.
Recently, attention has been given to thermoplastic saturated norbornene polymers such as hydrogenated products of ring opening polymers of norbornene monomers and addition polymers of norbornene monomers and ethylene as optical plastic materials such as optical disc substrate. (Japanese Patent Kokai Nos. Sho 60-26024 and 63-31752 and Hei 1-24826, EP 303246 and EP 317262, and U.S. Pat. No. 4614778).
These thermoplastic saturated norbornene polymers are a kind of polyolefin polymers and have excellent water resistance, chemical resistance and solvent resistance like other polyolefin polymers and besides, polymers having a high heat resistance of higher than the glass transition temperature 100.degree. C can be synthesized. Furthermore, they have a total light transmittance of 90% or higher and thus, are superior in transparency.
Therefore, thermoplastic saturated norbornene polymers not only have the possibility of being used for various uses as polyolefin materials having heat resistance, but also have the possibility of being developed for optical uses as transparent materials having both the low birefringence and excellent transparency, water resistance and heat resistance.
However, these thermoplastic saturated norbornene polymers are brittle as compared with polyethylene, polypropylene and PMMA and, when extrusion formed into sheet, film, and rod, are apt to crack or break and articles of sufficient strength have not been obtained. There are further problems that a bubble-like defect is often generated in extrusion formed articles, which appears on the surface in the form of visible streaks or which reduces strength. Even if visible large streaks are not present, in many cases, observation in detail under light microscope reveals invisible fine voids or crack-like defects (hereinafter referred to as "microvoid") generated inside of the molded articles. Thus, hitherto, extrusion formed articles of thermoplastic saturated norbornene polymers with no microvoids have not been obtained.
As other method for obtaining sheet-like or film-like articles, there is a casting method which comprises casting a polymer as a solution in a suitable solvent and evaporating the solvent and according to this method there are obtained articles having substantially no microvoids. However, even if it is a thin article of a few .mu.m in thickness, solvent cannot be completely evaporated and a few % of solvent remains and sufficient strength cannot be obtained. Furthermore, the remaining solvent gradually evaporates during using the article to change properties of the article or the evaporated solvent adversely influences other parts provided around the article. From the point of endurance, film-like or sheet-like articles are required to have a tensile strength of at least 800 kg/cm.sup.2, preferably at least 900 kg/cm.sup.2, but those which have such sufficient strength have not been obtained by casting method.
Therefore, hitherto, there have not yet been provided sheet-like or film-like articles comprising a thermoplastic saturated norbornene polymer which have no defects such as microvoids and cracks and have sufficient strength of at least 800 kg/cm.sup.2 in tensile strength.